FIG. 1 shows a schematic block diagram of a prior art linear voltage regulator (100) with high power supply rejection (PSR). As shown in FIG. 1, the feedback network (106), including a resistor divider, an error amplifier (112) and an optional extra amplifier (113), regulates the DC output voltage Vout (102) to a desired level given by: Vout=Vref*(1+R2/R1). The resistor R2 may be a short circuit, and the resistor R1 may be an open circuit in some implementations. The pass transistor Mpass (103) may be a field effect transistor (FET), a bipolar transistor, an LDMOS transistor or a FinFET device, and Mpass may be of either n-type or p-type. High-gain amplifiers are typically used as the implementation of the error amplifier (112) in the feedback network (106). The second stage of the feedback network (106) is optional and might provide gain higher than 1 or be used as a buffer stage to drive the pass transistor (103). The feed-forward block (105) is used to enhance the power supply rejection of the linear voltage regulator (100). Linear voltage regulator architectures are generally categorized into two main categories: Voltage regulators that require an external capacitor for compensation and voltage regulators that do not require an external capacitor for compensation. This last category is named “cap-less linear voltage regulators”. The compensation capacitor whether internal or external is not shown in FIG. 1 (100). Power supply rejection (PSR) is the ability of the voltage regulator to reject any noise coming from the supply through the Vin terminal in FIG. 1. Throughout this disclosure, the terms, “power supply,” “supply,” “Vin,” and “Vin terminal” may be used interchangeably to refer to the power source input to a voltage regulator.
One of the main features of linear voltage regulators (LVRs) is their good power supply rejection (PSR). At low frequencies, the PSR is dominated by the DC gain of the loop. At high frequencies, due to bandwidth limitations of the error amplifier, the PSR of the LVR suffers. A brute force technique to keep a high PSR would be to compromise between power consumption and the PSR. However, this is not a favorable strategy.
One of the techniques to improve power supply rejection in LVRs is to use a feed-forward (FF) block (105) through a FF path (110) to sense the input supply variations and cancel it at the output of the LVR. The FF block (105) consists of an active filter to equalize the input ripples to match the frequency response of the Direct Path (109). The adder (116) combines the direct path (109) and the FF path (110) to cancel the effect of the input ripples coming from the direct path. This technique improves the PSR dramatically at high frequencies.
The FF block (105) typically consists of active filters to shape the frequency content of the input ripples to match that of the direct path (109). These active filters serve also to boost the DC gain of the FF signal to match the DC gain of the direct path. The FF block consumes extra power consumption that increases the quiescent power consumption of the overall LVR.
Using a feed-forward (FF) cancellation path to reduce the effect of the input supply ripples has been proposed before, see El-Nozahi et al., “High PSR low drop-out regulator with feed-forward ripple cancellation technique,” Solid-State Circuits, IEEE Journal, Vol. 45, no. 3 (2010): pp. 565-577. The technique disclosed in this paper uses an FF cancellation path that involves an active filter. The power consumption of the active filter is proportional to the frequency range to be covered by the FF block. The technique was introduced for a low drop-out (LDO) linear voltage regulator (LVR) that uses an external capacitor for stability purposes.
In U.S. Patent Application Publication No. 2012/0212199 A1, by Amer et al., an input FF cancellation technique was disclosed that works with cap-less LDOs. The FF block is also active and hence consumes extra power.
While these prior art approaches provide useful improved power supply rejection for voltage regulators, there is still a need for better approaches.